As a method for imparting water/oil repellency to the surface of an article (such as a fiber product), a method of treating the article by means of a water/oil repellent composition composed of an emulsion having, dispersed in a medium, a copolymer having structural units based on a monomer having a polyfluoroalkyl group having at least 8 carbon atoms (a polyfluoroalkyl group will be hereinafter referred to as a Rf group), has been known.
However, recently, EPA (US Environmental Protection Agency) has pointed out that a compound having a perfluoroalkyl group (a perfluoroalkyl group will be hereinafter referred to as a RF group) having at least 8 carbon atoms is likely to be decomposed in the environment or in vivo, and the decomposition product is likely to be accumulated, i.e. it presents a high environmental impact. Therefore, a copolymer for a water/oil repellent composition has been required which has structural units based on a monomer having a Rf group having at most 6 carbon atoms, wherein structural units based on a monomer having a Rf group having at least 8 carbon atoms are reduced as far as possible.
As a water/oil repellent composition containing such a copolymer, the following water/oil repellent composition has been, for example, proposed.
A water/oil repellent composition comprising a copolymer having structural units based on the following monomer (a), structural units based on the following monomer (b) and structural units based on the following monomer (c), and a medium (Patent Document 1):
Monomer (a): A monomer having a RF group having at most 6 carbon atoms.
Monomer (b): A (meth)acrylate having an alkyl group having from 20 to 30 carbon atoms.
Monomer (c): Vinylidene chloride
However, an article treated with such a water/oil repellent composition is insufficient in water repellency against water fallen from a height (rain fall) (hereinafter referred to as dynamic water repellency) and in water repellency after drying without forcibly heating after washing (hereinafter referred to as post-air-drying water repellency).